1. Field of the Invention
This invention relates to a method of preparing stable alpha-olefin polymers and more particularly to a method of preparing alpha-olefin polymers which are stable against thermal oxidation and ageing. Still more specifically, this invention is directed to stabilizing polypropylene by treating the polymerization slurry thereof with small but effective amounts of a surface-active agent consisting essentially of a fatty acid ester of the condensation product of ethylene oxide and sorbitan. The invention is also directed to the stabilized polymers produced by such treatment.
2. Description of the Prior Art
The alpha-olefin polymers and their method of polymerization are known. More specifically, alpha-olefins may be polymerized in the presence of catalysts consisting of at least one organometallic compound of a metal belonging to Groups I, II or III of the Periodic Table and a solid crystalline halide of a transition metal belonging to Groups IV, V or VI of the Periodic Table. In addition to the catalyst, there may be present during the polymerization an activating substance such as a pyridine, a phosphine, an amine, a bismuthine, and/or an ether. The monomer is usually introduced into a solvent which acts as a suspending agent for the polymerization product. These solvents are inert with respect to the catalyst which may be, e.g., an aluminum alkyl and a titanium trichlo ride. The polymerization is normally carried out for a period of time ranging from 10 to 100 hours at a temperature ranging from about 30.degree. to 90.degree.C under a pressure of 1-15 atmospheres. The slurry obtained at the end of the polymerization is then subjected to treatment with a purifying agent in order to reduce the ash content of the product. Subsequently, the slurry of polymer is treated with a surface-active agent and then stripped or centrifuged to remove the solvent.